1. Field of the Invention
The invention relates generally to activation of a C—H bond using a low temperature plasma and more specifically to activation of a C—H bond using a low temperature plasma and an inlet liquid stream.
2. Description of the Related Art
Electrical discharge plasma contacting liquid phases has been studied for a wide range of chemical, biomedical, environmental, and materials synthesis applications. The synthesis of a number of organic and inorganic compounds by gas-liquid plasma can involve glow discharge electrolysis whereby one electrode is placed inside the liquid phase and one in the gas phase. A wide range of other gas-liquid contacting schemes has been studied including falling films, aerosol sprays, and bubble injection into liquids. It has been shown that the presence of the liquid phase not only affects plasma properties such as electron energy and density, but also the chemical reactions which take place. The liquid phase can also serve as a source of additional vapor phase reactant as well as function as a reservoir to collect the generated products, protecting those products from degradation by direct electron attack in the gas phase plasma. Reactions with organic compounds in plasma discharges have been investigated for a wide range of applications and conditions including cases of plasma polymerization, plasma discharge in organic liquids, and the more commonly studied cases of organic compounds in liquid water for pollution control. Plasma generated directly in an organic liquid phase has been demonstrated to form diamond coatings and other carbonized materials such as nanofibers. Gas phase plasma (spark discharge: 3 to 12 W) generated with argon over heavy oils (n-C10 to n-C25 hydrocarbons) leads to significant chain breakage to form one to four carbon containing compounds with ethylene and hydrogen being the predominate species. Liquid n-hexadecane was studied as a model of a hydrocarbon oil and was cracked into C6 to C15 hydrocarbons using a dielectric barrier discharge with a methane carrier over the liquid hydrocarbon. In another example, crude oil was treated with a dielectric barrier discharge for various carrier gases (H2, CO2, CH4) where rheological analysis showed a decrease in viscosity of the crude oil treated by plasma, and NMR analysis showed that the plasma treatment primarily led to water extraction from the naturally occurring emulsified water in the crude. Finally, an 80 W microwave plasma with water vapor over a heavy oil liquid demonstrated a series of reaction products from long chain aromatics to linear and shorter aromatic rings and, finally, syngas, CO2 and small alkanes and alkenes, as well as traces of other carbonaceous products.
Previous studies have also demonstrated efficient production of H2 from methanol and water/methanol mixtures, as well as other alcohol solutions, using a spray reactor. Clearly at high enough plasma power and exposure time, a wide range of hydrocarbons, even from heavy oils, can be cracked to relatively small compounds. The key issues that will make these types of applications useful for chemical synthesis of valuable products are to control or stop the plasma-induced radical reactions and to promote reaction selectivity. For example, some selectivity was demonstrated in a gas phase microwave plasma with n-hexane vapor in flowing argon through changes in the plasma input power, feed flow rates, and location of the feed.
Oxidation of the C—H group in alkanes under low temperature and pressure conditions is a significant challenge due to selectivity issues and over oxidation by harsh conditions. While catalysts have been developed that use hydrogen peroxide to form OH radicals capable of functionalizing alkanes, the reactions are quite complex. Hydrogen abstraction of alkanes at high temperature primarily for combustion has also been studied.
Plasma processes have been demonstrated to produce methanol from methane with high efficiency. Much of the extensive literature on methane conversion in plasma reactors focuses on methane conversion in dry gas to higher hydrocarbons and some effort has been devoted towards methane to methanol and/or formaldehyde conversion with water vapor and or liquid water films.
In plasma discharge in humid gas the direct conversion of methane to methanol can be expressed by Equation (1):CH4+H2O→CH3OH+H2  (1)The conversion proceeds by the direct reaction of methyl radicals, CH3, with hydroxyl radicals, OH. In addition to methanol, formaldehyde and formic acid are formed. Using a 500 Hz pulsed discharge reactor at approximately 400 degrees Celsius and relatively low pressure of 10 to 40 Torr, and power of 2 to 6 W, they found methanol yield of approximately 0.8% with energy yields of up to 10 g/kWh for glow-like discharge, but at high voltage spark-like discharge with lower power (5 mW) discharge they claim approximately 100 times better efficiency at 1 kg/kWh. While the yield is relatively small, the energy efficiency is high and may be economically competitive. The reaction kinetics of methane oxidation have been extensively studied and include the main reactions given by Equations (2)-(6):CH4+OH→CH3+H2O  (2)CH3+O2(+M)→CH3OO(+M)  (3)CH3+HO2→CH3O+OH  (4)CH3OO+CH3→CH3O+CH3O  (5)CH3O+CH4→CH3OH+CH3  (6)
As with the formation of hydrogen peroxide and hydrazine, the formation of methanol may be optimized under conditions where degradation reactions with radicals are minimized and over oxidation to CO and CO2 is suppressed.
Alkanes and other compounds have been oxygenated by oxygen radicals in oxygen plasma as well. However, oxidation with hydroxyl radicals from liquid water in gas-liquid plasma systems has mostly been used to oxidize organic compounds in liquid water for pollution control. Reactions of alkanes such as n-hexane and cyclohexane with OH radicals produced from liquid water by plasma discharge where the plasma channels propagate along a gas-liquid interface have not, to our knowledge, been reported.
There are three important differences between the functionalization of hydrocarbons to produce small intermediate products by plasma and the more extensively studied plasma polymerization processes. In plasma polymerization, the desired goal is to form a surface polymer coating using gas phase plasma containing the precursor molecules. In such cases, a large conversion is required to form the coating. In order to produce a large conversion, a large plasma energy is required which leads to complete dissociation of the precursor compounds into small organic fragments. The resulting recombination reactions are not significantly selective due to the large number of possible reactions which can occur. One goal of the present work is to introduce selectivity. Although selectivity may come at the cost of lower conversion, this cost can be compensated in synthetic chemistry by component recirculation as well as series or parallel reactor designs. The second issue relates to the site of the main polymerization reactions. In plasma polymerization there is still debate on whether the main polymerization reactions occur in the gas phase or on the surface. Both cases are predicted to lead to the “irregular structure” of the polymer, where the reactor pressure and plasma pulsing can affect the location of these reactions. In gas-liquid plasma systems the physical location of the plasma chemical synthesis will depend, in part, on the volatility of the precursor molecule. Under conditions of low volatility, the plasma radicals may directly impinge on the liquid surface initiating reactions at the interface or even generate some radicals in the liquid phase. For high volatility cases, the organic liquid is fully vaporized and can react directly in the gas phase. Different product distributions are expected in these different conditions. A third issue relates to modification of reactor/reaction conditions involving generation of pulses by the power supply. Shorter plasma pulses (or with superimposed pressure pulses) have been shown to control chain propagation in plasma polymerization, but again at the cost of yield.
There is a need to utilize a pulsed plasma reactor with a flowing liquid water film, carrier gas, and various organic compounds for the synthesis of more chemical species.